LV Short Circuit Calculator (IEC 60909)

Source (Transformer)

kVA
%

Downstream Cable

m
no.

Fault Calculation

Rated Current (In) - A
Isc @ Transformer - kA
Total Impedance (Ztot) - mΩ
Isc @ Load End - kA

*Calculated using IEC 60909 Voltage Factor c = 1.05.
Assumes infinite upstream primary bus.

Technical Notes & Engineering Logic

Variable Definitions

  • Isc: Symmetrical RMS Short Circuit Current.
  • Sr: Transformer Rating (kVA).
  • uk%: Transformer Short Circuit Impedance.
  • Ztot: Total impedance (Transformer + Cable).

Formulas

  • Isc(tx): (c × In) / (uk% / 100)
  • Vector Addition: Ztot = √(RcableΒ² + XtxΒ²)
  • Isc(load): (c × VLL) / (√3 × Ztot)

Assumptions

  • Impedance: Transformer is treated as pure Reactance (X). Cable is treated as pure Resistance (R).
  • Cable Temp: R values at 20Β°C (conservative for maximum fault current).
  • Source: Upstream Grid Impedance is ignored (Infinite Bus).
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